1. Field of the Invention
The present invention relates to an inkjet print method and an inkjet print apparatus by which an image is printed on a print medium by using an inkjet print head having nozzle arrays each including ink-ejecting nozzles densely arranged.
2. Description of the Related Art
With the spread of information processing devices and communication devices, such as computers and word processors, there are more and more demands for output devices that output digital image information processed by such devices. As one of such output devices, an inkjet print apparatus that forms dots on a print medium through ejection of ink droplets is rapidly spreading. Recent demands increasingly call for output devices capable of outputting color print images. Since output of photograph images particularly requires high image quality, reduction in volume of each ink droplet is anticipated.
Meanwhile, with the recent technical development in integrated arrangement of nozzles, production of high-density, long print head is getting possible. However, print operations using such high-density, long print head can cause the following problem. Specifically, high speed print-head scanning or print-medium scanning generates an air current between the print head and the print medium. In addition, simultaneous ejection of many ink droplets from the print head having nozzles densely arranged generates turbulence in the air current. As a result, landing positions of the ink droplets are shifted from their desired positions, drastically deteriorating the image quality.
A technique disclosed by Japanese Patent Laid-Open No. 2006-192892 is known as a method for preventing such image quality deterioration. In Japanese Patent Laid-Open No. 2006-192892, a print is performed using mask patterns in which image data is thinned out. Specifically, image data to be printed is thinned out with different high and low thinning percentages alternately in an arrangement direction of the nozzles. Printing on the same area multiple times by using such mask patterns reduces the turbulence of the air current generated between the print head and the print medium, and allows the landing position of each ink droplet to be kept accurate. Thereby, a high quality image can be obtained.
In the technique described in Japanese Patent Laid-Open No. 2006-192892, an enough space for allowing the air current to escape is formed between walls of ejected ink. Accordingly, high-print-percentage areas and low-print-percentage areas need to have lengths appropriate for forming such space, and therefore have such sizes that a concentration difference in the pattern in one scan is visually distinguishable. An image in each area is formed by images by different scans compensating for each other. Accordingly, when the images are printed with a reciprocating scan, a scan start portion and a scan end portion overlap each other in left-edge and right-edge parts of the images.
For example, if a print of an image in the same area is completed with two scans, a scan start portion of a first scan and a scan end portion of a second scan overlap and compensate for each other, focusing on the edge parts of the image. Generally, at the start of the scans, the temperature of the print head is as low as approximately an ambient temperature because of a standby time for, for example, feeding the print medium between the scans. Meanwhile, at the end of the scans, the print head has an elevated temperature because of the ejection operations during the scans. For this reason, a high-print-percentage area printed at the start of the scans and a high-print-percentage area printed at the end of the scans with the print head having an elevated temperature have different print concentrations due to the temperature change of the print head. Thus, concentration unevenness tracing the mask patterns is reflected in the printed image in some cases.